1. Field of the Invention
This invention relates to providing a light source for analytical purposes that is stable with time, temperature and any variation in electrical connecting means.
2. Description of Related Art
Optical absorption spectroscopy is a measurement technique for identifying and quantifying known as well as unknown substances. The measurement is typically performed by relating the amount of light from a light source passing through or reflecting off of the sample under test, with that same light passing through or reflecting off a reference sample. A wavelength-selecting device is placed in the optical path either before the detector or after the light source to limit the spectrum to a specific wavelength region or regions necessary to perform the measurement. The light source is typically an electrically heated element, which produces optical emission proportional to the temperature to which it is heated. The measurement accuracy is then limited by the thermal stability of the light source.
Due to the nature of some optical detectors, which are only sensitive to changes in optical energy, it is sometimes necessary to modulate the light source in order to produce an electrical output from the detector. Modulation can be done in one of three ways, mechanically, optically or electronically. Electronic modulation is the subject of this invention.
To electronically modulate a light source, current passing through it is periodically interrupted or is itself modulated to vary the electrical power delivered to the light source and in so doing vary its temperature. This technique is often referred to as xe2x80x9cpulsingxe2x80x9d. Since the temperature is intentionally varied in pulsing, thermal stability issues become more complex. During pulsing, the source cycles between a high and low temperature. The stability of both of these points as well as their timing now become factors affecting the accuracy and stability of the optical measurement. Primary factors affecting the absolute value of the high and low temperatures are as follows;
1. Ambient temperature;
2. Mass of the heated element;
3. Thermal resistance or heat sinking properties of the source structure and surrounding atmosphere;
4. Element electrical resistance; and
5. Electrical power delivered to the element;
The focus of this invention lies in controlling the effect of pulsing, i.e., the frequency and waveform of the pulsing, on the elements electrical resistance and the electrical power delivered to the element.
The present invention is a method of modulating an optical light source comprising: (a) feeding a solid state voltage reference into a non-inverting input of an operational amplifier through a resistive divider to provide an output from said amplifier; (b) passing the output of said operational amplifier through a series pass transistor which regulates the current from a power supply to the optical light source and to provide a voltage across said light source; (c) feeding said voltage across said light source back to an inverting input of said operational amplifier to provide a feedback signal; (d) generating a clock signal with a crystal-controlled oscillator; (e) shorting the non-inverting input of said operational amplifier to ground synchronously with said clock signal; and (f) taking said feedback signal provided in step (c) through separate wires to the terminals of said light source thus eliminating the adverse effect of cable and connector contact resistances.